Dinitrotoluene is conventionally manufactured by nitrating toluene with a mixture of nitric and sulfuric acids. The crude dinitrotoluene thus obtained contains the desired 2,4- and 2,6-dinitrotoluene isomers along with small amounts, usually less than 5% of isomeric dinitrotoluene impurities, notably the 2,3-, 2,5- and 3,4-dinitrotoluene isomers (so-called "meta" isomers). Dinitrotoluene is valuable as an intermediate in the manufacture of 2,4,6-trinitrotoluene (TNT) employed as an explosive for military use. The TNT obtained by nitration of crude dinitrotoluene contains the desired 2,4,6-isomer along with a small amount, usually about 3-5% of the undesired 2,4,5-, 2,3,4, and 2,3,6-trinitrotoluene isomers, which result from the nitration of the aforesaid "meta" isomers present in crude dinitrotoluene. At present TNT is purified by treatment with a hot solution of sodium sulfite (sellite) solution, which removes the undesired TNT isomers but results in a very concentrated and intensely reddish colored solution whose diposal is costly and contributes to a serious pollution of streams into which it is discharged.
It is thus evident that the removal of the so-called meta isomers in crude dinitrotoluene is highly desirable, since it would permit the production of military grade 2,4,6-TNT directly and thus eliminate the need for sellite purification and attendant purification problems.